Carlson, in U.S. Pat. No. 4,064,521, incorporated herein by reference, has disclosed the fabrication of semiconductor devices incorporating hydrogenated amorphous silicon (a-Si:H) deposited onto a substrate from a glow discharge. Typically these films are deposited on a substrate positioned in the cathodic or anodic dark region of the discharge. While excellent photovoltaic devices have been fabricated, the area of the devices which can be fabricated and the throughput rate in the deposition apparatus are limited. Recently useful photovoltaic devices which include a-Si:H have been fabricated with a high production rate for the a-Si:H layer by positioning the substrate in the positive column of the glow discharge. The production rate is higher since, although the deposition rate is low due to the small electric field, the volume of the discharge in which deposition can occur is much larger.
During the deposition of a layer from the glow discharge, the growing film, enveloped in the discharge, may be subject to bombardment by electrons, ions and energetic neutrals. Such bombardment has been shown to have profound effects on the properties of an a-Si:H film including the growth rate, film structure, composition, and purity content, density of defects and optical transport properties.
I have found that during the deposition of a-Si:H in the positive column a plasma potential gradient develops relative to a conductive substrate on which the layer is deposited when the plane of the conductive substrate is oriented parallel to the positive column. This relative plasma potential ranges from small negative to high positive values and its magnitude varies approximately linearly with the substrate length. This plasma potential variation is determined by the electric field, E, which exists in the positive column and which is of the order of 1-10 volts per centimeter. When the plasma potential relative to the substrate is low (-5 to 10 volts) during the deposition of the a-Si:H layer, the resulting film shows a near maximum photovoltaic performance. As this plasma potential increases progressively outside of this range for either voltage polarity, nonuniformities in film thickness along the substrate and a drastic decrease in the photovoltaic performance result.
Deposition of the semiconductor material in the positive column of the glow discharge offers the potential of large economies in the manufacturing process. Thus it would be desirable to improve the deposition method to compensate for eliminating these deficiencies in the positive column method while retaining its advantage of high throughput.